Lead condition assessment for an implantable medical device

ABSTRACT

A method, system, and apparatus for performing a lead condition assessment and/or a lead orientation determination associated with an implantable medical device (IMD). A first impedance is determined. The first impedance relates to the impedance relative to a first electrode and a portion of the IMD. A second impedance is determined. The second impedance relates to the impedance relative to a second electrode and the portion of the IMD. The first impedance is compared with the second impedance to determine an impedance difference. A determination is made whether the impedance difference is outside a predetermined tolerance range. Furthermore, artifact measured during impedance measurements or test pulses may be compared to assess lead orientation. An indication of a lead condition error is provided in response to determining that the impedance difference is outside the predetermined tolerance range.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to implantable medical devices, and,more particularly, to methods, apparatus, and systems for providing anassessment relating to the condition of a lead coupled to theimplantable medical device.

2. Description of the Related Art

There have been many improvements over the last several decades inmedical treatments for disorders of the nervous system, such as epilepsyand other motor disorders, and abnormal neural discharge disorders. Oneof the more recently available treatments involves the application of anelectrical signal to reduce various symptoms or effects caused by suchneural disorders. For example, electrical signals have been successfullyapplied at strategic locations in the human body to provide variousbenefits, including reducing occurrences of seizures and/or improving orameliorating other conditions. A particular example of such a treatmentregimen involves applying an electrical signal to the vagus nerve of thehuman body to reduce or eliminate epileptic seizures, as described inU.S. Pat. No. 4,702,254 to Dr. Jacob Zabara, which is herebyincorporated in its entirety herein by reference in this specification.Electrical stimulation of the vagus nerve (hereinafter referred to asvagus nerve stimulation therapy or VNS) may be provided by implanting anelectrical device underneath the skin of a patient and performing adetection and electrical stimulation process. Alternatively, the systemmay operate without a detection system once the patient has beendiagnosed with epilepsy, and may periodically apply a series ofelectrical signals to the vagus (or other cranial) nerve intermittentlythroughout the day, or over another predetermined time interval.

Generally, therapeutic electrical stimulation is delivered by theimplantable device via a lead, which is coupled to one or moreelectrodes coupled, in turn, to a target location of the patient's body.A plurality of electrodes that are associated with an implantablemedical device are generally operatively connected to the implantabledevice via individual leads. A number of leads may project from theimplantable device onto various portions of a patient's body. Forexample, a number of electrodes may be attached to various points of anerve or other tissue inside a human body. Occasionally, problems withthe lead and/or electrodes may occur. These problems may include amalfunction or damage of the lead and/or electrode, or a change in thetissue surrounding the implanted lead and/or electrode.

Often, various electrodes are implanted in contact with target portionsof the human body, such as a vagus nerve, in order to deliver electricalsignals to provide therapy or to monitor signals. Subsequent to theimplanting of the implantable device, the associated leads, electrodesand/or connections between the electrodes and the implantable device maydeteriorate over time. Additionally, changes in the tissue surroundingthe lead and/or electrodes may cause electrical variations experiencedby the implantable device system, which may affect the operation of theleads and electrodes themselves. Electrical characteristics associatedwith the leads and electrodes carrying the stimulation signal ormonitored signal may deteriorate over time, thereby altering theoperation of the implantable device. Furthermore, physiologic changes inthe human body may also affect the operation of the implantable devicesince these changes may affect the electrical characteristicsexperienced by the lead and/or the electrodes.

State-of-the-art assessment of lead condition may include measuring animpedance between a plurality of electrodes[r1]. A rise in the leadimpedance may provide an indication that the lead condition has changed.This may be caused by various factors, such as deterioration of thelead, deterioration of the electrode, deterioration of connectionsbetween the electrode and the implantable device, and/or thephysiological changes in the human body. Based upon the impedancemeasurements, state-of-the-art technology calls for assessing orconcluding that there may be lead problems. However, a simple rise inlead impedance may not necessarily reflect actual lead problems. Forexample, physiological impedance changes may provide a false negativeindication that there are lead problems. Additionally, the lack of arise in lead impedance may provide a false positive indication thatthere are no problems with the leads or electrodes. For example, a leadproblem may be masked by an apparent lack of change in lead impedance.This apparent lack of change in the lead impedance may actually be anincrease in lead impedance (e.g., due to lead/electrode damage) beingmasked by a reduction in the physiologic impedance. The reduction in thephysiologic impedance may counter-balance the rise in the electrode orlead impedance that may have been the result of actual damage. However,the result causes a false assessment of the actual condition of the leadand/or electrode. This could lead to improper delivery of therapeuticstimulation or improper assessment of monitored signals by theimplantable device.

Other problems with the state-of-the-art include the fact that theinsertion or placement of the lead and electrodes into the patient'sbody may be implemented incorrectly. For example, the insertion of theleads may be reversed compared to the originally intended position ofthe electrodes and/or leads. For example, the lead/electrode in a setthat was originally intended to be positioned proximal to theimplantable device may be inadvertently positioned in a distal position,while the intended distal electrode may inadvertently become theproximal electrode. Therapy stimulation being provided may beineffectively administered or monitored signals may be errantly assesseddue to the various errors described herein.

The present invention is directed to overcoming, or at least reducing,the effects of one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In one aspect, the present invention comprises a method for determininga condition of a lead assembly coupled to an implantable medical device(IMD). A first impedance is determined relative to a first electrode anda reference electrode. The reference electrode may comprise a portion ofthe IMD. A second impedance is determined relative to a second electrodeand the reference electrode. The first impedance is compared to thesecond impedance to determine an impedance difference. A determinationis made whether the impedance difference is outside a predeterminedtolerance range. An indication of a lead condition error is provided inresponse to determining that the impedance difference is outside thepredetermined tolerance range. In another aspect, the present inventioncomprises a method for determining a condition of a lead assemblyassociated with an implantable medical device (IMD). The methodcomprises determining a first impedance relative to a first electrodeand a reference electrode. A second impedance is determined relative toa second electrode and the reference electrode. A third impedance isdetermined relative to the first electrode and the second electrode. Themethod further comprises comparing the first impedance to the secondimpedance to determine an impedance difference. A determination is madewhether the impedance difference is outside a predetermined tolerancerange. The method additionally comprises providing an indication of alead condition error in response to determining that the impedancedifference is outside a predetermined tolerance range, and comparing theimpedance difference to the third impedance. Based on the comparison ofthe impedance difference to the third impedance, a source of the leadcondition error is identified. The source may include the firstelectrode, the second electrode, or a physiological impedance.

In another aspect, the present invention comprises a method fordetermining a condition of a lead assembly associated with animplantable medical device (IMD). The method comprises providing a firsttest signal to a first electrode coupled to the IMD through a firstlead, and measuring a first signal artifact relating to a secondelectrode coupled to the IMD through a second lead. The first signalartifact results from the first test signal being applied to the firstelectrode. A second test signal is provided to the second electrode, anda second signal artifact relating to the first electrode is measured.The second signal artifact results from the second test signal beingapplied to the second electrode. The method further involves comparingthe first signal artifact to the second signal artifact to determine asignal artifact differential. A determination is made as to whether thesignal artifact differential is outside a predetermined tolerance range.Finally, the method includes providing an indication of a lead conditionerror in response to determining that the signal artifact differentialis outside the predetermined tolerance range.

In yet another aspect, the present invention comprises a method fordetermining an orientation of a lead assembly associated with animplantable medical device (IMD). The method involves determining afirst impedance relative to a first electrode and a reference electrode,as well as a second impedance relative to a second electrode and thereference electrode. The first impedance is compared to the secondimpedance to determine whether the first impedance is greater than thesecond impedance. Based on the comparison of the first and secondimpedances, the method comprises determining which of the firstelectrode and the second electrode is positioned distal to the referenceelectrode. In another aspect, the present invention comprises a methodfor determining the orientation of a lead assembly associated with animplantable medical device (IMD). A first test signal is provided to afirst electrode coupled to the IMD through a first lead, and a firstsignal artifact relating to a second electrode coupled to the IMDthrough a second lead is measured. The first signal artifact resultsfrom the first test signal being applied to the first electrode. Asecond test signal is provided to the second electrode, and a secondsignal artifact relating to the first electrode is measured. The secondsignal artifact results from the second test signal being applied to thesecond electrode. The first signal artifact is compared to the secondsignal artifact to determine whether the first signal artifact isgreater than the second signal artifact. A determination is made as towhich of the first electrode and the second electrode is distal to theIMD in response to comparing the first and second signal artifacts.

In another aspect, the present invention comprises a system forperforming a lead condition assessment and/or a lead orientationdetermination associated with an implantable medical device (IMD). Thesystem includes an implantable medical device (IMD) for delivering anelectrical signal to a patient's body; a first electrode coupled to theIMD and to a first portion of a patient's body; a second electrodecoupled to coupled the IMD and to a second portion of a patient's body;and an external device to communicate with the IMD. The system comprisesa controller to determine a first impedance relative to a firstelectrode and a reference electrode. The controller is also adapted todetermine a second impedance relative to a second electrode and thereference electrode. The controller is also adapted to compare the firstimpedance and the second impedance to determine an impedance difference,and to determine whether the impedance difference is outside apredetermined tolerance range. The system is also adapted to provide anindication of a lead condition error in response to determining that theimpedance difference is outside the predetermined tolerance range. Inanother aspect, the present invention comprises a system for performinga lead condition assessment and/or a lead orientation determinationassociated with an implantable medical device (IMD). The system includesan implantable medical device (IMD) for delivering an electrical signalto a patient's body; a first electrode coupled to the IMD and to a firstportion of the patient's body; a second electrode coupled to the IMD andto a second portion of the patient's body; and an external device tocommunicate with the IMD. The IMD includes a stimulation unit toproviding a first test signal to a first electrode coupled to the IMDthrough a first lead and to provide a second test signal to a secondelectrode coupled to the IMD through a second lead. The IMD alsoincludes a signal unit to measure a first signal artifact relating to asecond electrode coupled to the IMD through a second lead. The firstsignal artifact results from the first test signal being applied to thefirst electrode. The signal unit is also adapted to measure a secondsignal artifact relating to the first electrode, which results from thesecond test signal being applied to the second electrode. The IMD alsoincludes a controller to compare the first signal artifact to the secondsignal artifact to determine whether the first signal artifact isgreater than the second signal artifact and in response, to determinewhich of the first and the second electrodes is distal to the IMD. TheIMD also includes a communication unit to communicate data relating to alead orientation of the first and second leads to the external devicebased upon the determination the first electrode is in at least one of adistal and a proximal position.

In yet another aspect of the present invention, a computer readableprogram storage device encoded with instructions is provided forperforming a method for determining a condition of a lead assemblyassociated with an implantable medical device (IMD). The computer,performs a method which comprises: determining a first impedancerelative to a first electrode and a reference electrode; determining asecond impedance relative to a second electrode and the referenceelectrode; comparing the first impedance to the second impedance todetermine an impedance difference; determining whether the impedancedifference is outside a predetermined tolerance range; and providing anindication of a lead condition error in response to determining that theimpedance difference is outside the predetermined tolerance range. Inyet another aspect of the present invention, a computer readable programstorage device encoded with instructions is provided for performing alead condition assessment and/or a lead orientation determinationassociated with an implantable medical device (IMD). The computerperforms a method, which comprises: providing a first test signal to afirst electrode coupled to the IMD through a first lead; measuring afirst signal artifact relating to a second electrode coupled to the IMDthrough a second lead, the first signal artifact resulting from thefirst test signal being applied to the first electrode; providing asecond test signal to the second electrode; measuring a second signalartifact relating to the first electrode, the second signal artifactresulting from the second test signal being applied to the secondelectrode; comparing the first signal artifact to the second signalartifact to determine whether the first signal artifact is greater thanthe second signal artifact; determining which of the first electrode andthe second electrode is distal to the IMD in response to determiningwhether the first signal artifact is greater than the second signalartifact.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify like elements, and in which:

FIGS. 1A-1D provide stylized diagrams of an implantable medical deviceimplanted into a patient's body for providing stimulation to a portionof the patient's body, in accordance with one illustrative embodiment ofthe present invention;

FIG. 2 provides a layout depiction of an implantable medical device andassociated lead and electrodes, in accordance with one illustrativeembodiment of the present invention;

FIG. 3 provides a block diagram depiction of an implantable medicaldevice, in accordance with one illustrative embodiment of the presentinvention;

FIG. 4 provides a more detailed block diagram depiction of the signaldetection unit of FIG. 3, in accordance with one illustrative embodimentof the present invention;

FIG. 5 provides a block diagram depiction of an impedance unit of FIG.3, in accordance with one illustrative embodiment of the presentinvention;

FIG. 6 provides a flowchart depiction of a method of performing a leadcondition assessment, in accordance with one illustrative embodiment ofthe present invention;

FIG. 7 illustrates a flowchart depiction an alternative embodiment of amethod of performing the lead condition assessment;

FIG. 8 provides a flowchart depiction of the method of performing a leadorientation detection, in accordance with one illustrative embodiment ofthe present invention; and

FIG. 9 provides flowchart depiction of an alternative embodiment of amethod of performing the lead orientation detection.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described herein. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. In the development of any such actualembodiment, numerous implementation-specific decisions must be made toachieve the design-specific goals, which will vary from oneimplementation to another. It will be appreciated that such adevelopment effort, while possibly complex and time-consuming, wouldnevertheless be a routine undertaking for persons of ordinary skill inthe art having the benefit of this disclosure.

FIGS. 1A-1D depict a stylized implantable medical system 100 forimplementing one or more embodiments of the present invention. FIGS.1A-1D illustrate a signal generator 110 having a main body 112comprising a case, or shell 121, with an electrical connector 116 in aheader 114 (FIG. 1C) for connecting to lead assembly 122. The signalgenerator 110 is implanted in the patient's chest in a pocket or cavityformed by the implanting surgeon just below the skin (indicated by adotted line 145, FIG. 1B), similar to the implantation procedure for apacemaker pulse generator.

A stimulating electrode assembly 125, preferably comprising an electrodepair, is conductively connected to the distal end of the insulatedelectrically conductive lead assembly 122, which preferably comprises apair of lead wires (one wire for each electrode of an electrode pair).Lead assembly 122 is coupled at its proximal end to the electricalconnector 116 on header 114. The electrode assembly 125 is surgicallycoupled to the patient's tissue, e.g., a vagus nerve 127 in thepatient's neck. The present invention is suitable for use in implantablemedical devices connected to any body tissue, e.g., a pacemaker coupledto heart tissue. The electrode assembly 125 preferably comprises abipolar stimulating electrode pair (FIG. 1D), such as the electrode pairdescribed in U.S. Pat. No. 4,573,481 issued Mar. 4, 1986 to Bullara.

As used herein, the term lead assembly refers to the combination of thelead individually and the electrodes coupled thereto. Persons of skillin the art will appreciate that many electrode designs could be used inthe present invention. For embodiments of the present inventioninvolving vagus nerve stimulation, two electrodes are preferably wrappedabout the vagus nerve, and the electrode assembly 125 is preferablysecured to the nerve 127 by a spiral anchoring tether 128 (FIG. 1D) suchas that disclosed in U.S. Pat. No. 4,979,511 issued Dec. 25, 1990 toReese S. Terry, Jr., and assigned to the same assignee as the instantapplication. Lead assembly 122 is secured, while retaining the abilityto flex with movement of the chest and neck, by a suture connection 130to nearby tissue.

In one embodiment of the present invention involving nerve stimulation,the open helical design of the electrode assembly 125 (described indetail in the above-cited Bullara patent), which is self-sizing andflexible, minimizes mechanical trauma to the nerve and allows body fluidinterchange with the nerve. The electrode assembly 125 preferablyconforms to the shape of the nerve, providing a low stimulationthreshold by allowing a large stimulation contact area with the nerve.Structurally, the electrode assembly 125 comprises two electrode ribbons(not shown) of a conductive material such as platinum, iridium,platinum-iridium alloys, and/or oxides of the foregoing. The electroderibbons are individually bonded to an inside surface of an elastomericbody portion of the two spiral electrodes 125-1 and 125-2 (FIG. 1D),which may comprise two spiral loops of a three-loop helical assembly.The elastomeric body portion of each loop preferably comprises siliconerubber, and the third loop 128 (which typically has no electrode) actsas the anchoring tether 128 for the electrode assembly 125.

The lead assembly 122 may comprise two distinct lead wires or a coaxialcable whose two conductive elements are respectively coupled to one ofthe conductive electrode ribbons 125-1 and 125-2. One suitable method ofcoupling the lead wires or cable to the electrodes comprises a spacerassembly such as that disclosed in U.S. Pat. No. 5,531,778, althoughother known coupling techniques may be used.

In certain embodiments of the present invention, sensing elements may beused to provide data to the implantable medical system 100 concerningone or more body parameters. Although exemplary sensors are disclosedherein, persons of skill in the art will appreciate that the presentinvention is not limited to particular embodiments. Referring to FIG.1B, eye movement sensing electrodes 133 may be implanted at or near anouter periphery of each eye socket in a suitable location to sensemuscle movement or actual eye movement. The electrodes 133 may beelectrically connected to leads 134 implanted via a catheter or othersuitable means (not shown) and extending along the jawline through theneck and chest tissue to the signal generator 110. When included insystems of the present invention, the sensing electrodes 133 may beutilized for detecting rapid eye movement (REM) in a pattern indicativeof a disorder to be treated, as described in greater detail below.Alternatively or additionally, the electrodes in lead assembly 122 maybe used as sensing electrodes.

Alternatively or additionally, EEG sensing electrodes 136 may optionallybe implanted in spaced apart relation through the skull, and connectedto leads 137 implanted and extending along the scalp and temple and thento the signal generator 110 in the same manner as described above forthe eye movement electrode leads. Electrodes 133 and 136, or other typesof sensors may be used in some embodiments of the invention to triggeradministration of the electrical stimulation therapy to the vagus nerve127 via electrode assembly 125. Use of such sensed body signals totrigger or initiate stimulation therapy is hereinafter referred to as a“feedback” or “active” stimulation. Other embodiments of the presentinvention utilize a stimulation therapy delivered according to aprogrammed on/off duty cycle without the use of sensors to triggertherapy delivery. This type of delivery may be referred to as “passive,”“non-feedback,” or prophylactic stimulation. Both active and passivestimulation may be combined or delivered by a single IMD 200 accordingto the present invention. Either or both modes may be appropriate totreat the particular disorder diagnosed in the case of a specificpatient under observation. The therapeutic electrical signal may be acontinuous or pulsed signal; either type of signal may be appliedperiodically or intermittently to the vagus nerve.

The signal generator 110 may be programmed with an external computer 150(FIG. 1A) using programming software of the type copyrighted by theassignee of the instant application with the Register of Copyrights,Library of Congress, or other suitable software based on the descriptionherein, and a programming wand 155 may be used to facilitate radiofrequency (RF) communication between the computer 150 and the signalgenerator 110. The wand 155 and software permit noninvasivecommunication with the generator 110 after the latter is implanted. Thewand 155 is preferably powered by internal batteries, and provided witha “power on” light to indicate sufficient power for communication.Another indicator light may be provided to show that data transmissionis occurring between the wand and the generator.

A wide variety of stimulation therapies may be provided in implantablemedical systems 100 of the present invention. Different types of nervefibers (e.g., A, B, and C fibers being different fibers targeted forstimulation) respond differently to stimulation from electrical signals.More specifically, the different types of nerve fibers have differentconduction velocities and stimulation thresholds and, therefore, differin their responsiveness to stimulation. Certain pulses of an electricalstimulation signal, for example, may be below the stimulation thresholdfor a particular fiber and, therefore, may generate no action potentialin the fiber. Thus, smaller or narrower pulses may be used to avoidstimulation of certain nerve fibers (such as C fibers) and target othernerve fibers (such as A and/or B fibers, which generally have lowerstimulation thresholds and higher conduction velocities than C fibers).Additionally, techniques such as pre-polarization may be employedwherein particular nerve regions may be polarized before a more robuststimulation is delivered, which may better accommodate particularelectrode materials. Furthermore, opposing polarity phases separated bya zero current phase may be used to excite particular axons or postponenerve fatigue during long-term stimulation.

Embodiments of the present invention provide for assessing a lead and/oran electrode condition associated with an implantable medical devicesystem, which includes an implantable medical device, a plurality ofleads and a plurality of electrodes. Various electrodes andcorresponding leads may be implanted into a portion of a patient's body,such as a portion of a vagus nerve. The implantable medical device maybe coupled operatively to the electrodes via corresponding leads.Changing conditions in the human body, to the leads, and/or to theelectrodes, may cause a change in the operation of the implantablemedical device. These changes may affect the treatment of a patienthaving the implantable medical device. Embodiments of the presentinvention provide for monitoring the condition of the leads, electrodes,and/or the surrounding portions of the human body to assess thecondition of the leads and/or the electrodes. Various impedancecalculations and signal measurements associated with the various leadsand/or electrodes may be performed and analyzed to determine the leadconditions.

Additionally, embodiments of the present invention may be used toprovide a lead orientation detection. Various leads and electrodes maybe implanted in a predetermined orientation. A particular electrode maybe implanted as a distal electrode relative to the implantable medicaldevice or other reference electrode, wherein another electrode may beimplanted as a proximal electrode in relation to the implantable medicaldevice or other reference electrode. Embodiments of the presentinvention may be used to perform an automated check of the leadorientation and compare it with a predetermined, desired leadorientation. Upon an indication or detection that the implanted leadorientation varies from the predetermined lead orientation, theimplantable medical device may provide a warning or a message. Basedupon the message/warning relating to the lead orientation and/or thelead conditions provided by embodiments of the present invention, one ormore corrective actions may be performed by an external entity, such asa physician.

Turning now to FIG. 2, a block diagram depiction of an implantablemedical device 200, coupled to a first electrode 220 and a secondelectrode 230, is illustrated. The implantable medical device 200 (IMD)is capable of delivering a stimulation signal to a portion of thepatient's body (e.g., a portion of a vagus nerve). The electrodes may beaffixed to a portion of the patient's body, wherein the IMD 200 providesthe stimulation signal to the electrodes 220, 230. Alternatively, areference electrode independent from IMD 200 may be used instead of theIMD 200.

The first and second electrodes 220, 230, are coupled to the IMD 200 viacorresponding leads. In one embodiment, the first electrode 220 may bepositioned/implanted in a proximal orientation relative to the IMD 200.The second electrode 230 may be positioned in a distal orientationrelative to the IMD 200. Embodiments of the present invention providefor measuring various impedances in relation to the IMD and theelectrodes 220, 230. The IMD 200 may measure a first impedance 240,which is an impedance of the first electrode relative to a referenceelectrode that, in one embodiment, comprises the case 121 of the IMD200. The first impedance may be indicative of the lead impedance of thelead that couples the first electrode 220 to the IMD 200. The IMD 200may also measure a second impedance 260, which is the impedance of thesecond electrode 230 in relation to a reference electrode, e.g., thecase 121 of the IMD 200. This impedance may provide the lead impedanceof the lead that couples the second electrode 230 to the IMD 200.Further, the IMD 200 may measure the third impedance 250, which may bethe impedance of the first electrode 220 relative to the secondelectrode 230.

The various impedances described herein may be used to assess the leadorientation relative to an expected lead orientation and/or anassessment of various lead/electrode conditions. In one embodiment, thefirst impedance 240 and the second impedance 260 may be analyzed andcompared such that an impedance difference between the first and secondimpedances 240, 260, may be indicative of a particular lead condition.For example, if the difference between the first impedance and thesecond impedance exceeds a pre-determined threshold range or tolerancerange, a faulty lead condition may be deemed to have taken place. Thedifferences between the first impedance and the second impedance 240,260, may then also be analyzed in conjunction with the third impedance250 for further analysis of the lead condition. Additionally, theimpedances described in FIG. 2 may also be used to determine whether thelead orientation of the leads described in FIG. 2 complies with expectedlead orientation. Further details relating to the impedance measurementsare provided below.

In one embodiment, the lead impedances 240, 260, may be measured byemploying a test signal that may be sent to the first and/or the secondelectrodes via corresponding leads. The test signal may be of a varietyof types of signals, such as a current pulse signal. In one embodiment,the test signal may be a pulse signal of approximately 0.25 milliamps inamplitude with a pulse width of 130 microseconds. The resultant voltageassociated with the electrode under test may be measured. The currentand the voltage values may then be used to calculate the impedance.Additionally, when one electrode is exposed to a test pulse, a resultantartifact signal appearing on another electrode may be measured in orderto perform the lead orientation and/or the lead condition assessmentsdescribed herein. Additionally, the signal artifact may be analyzed tofurther evaluate a resistive characteristic and/or a capacitivecharacteristic.

Turning now to FIG. 3, a block diagram depiction of the IMD 200, inaccordance with one illustrative embodiment of the present invention isillustrated. The IMD 200 may be used for stimulation to treat variousdisorders, such as epilepsy, depression, bulimia, heart rhythmdisorders, etc. The IMD 200 may be coupled to various leads, e.g., 122,134, 137 (FIGS. 1A, 1B, 1D). Stimulation signals used for therapy may betransmitted from the IMD 200 to target areas of the patient's body,specifically to various electrodes associated with the leads 122.Stimulation signals from the IMD 200 may be transmitted via the leads122 to stimulation electrodes associated with the electrode assembly 125(FIG. 1A). Further, signals from sensor electrodes, e.g., 133, 136 (FIG.1B) associated with corresponding leads, e.g., 134, 137, may alsotraverse the leads back to the IMD 200.

The implantable medical device 200 may comprise a controller 310 capableof controlling various aspects of the operation of the IMD 200. Thecontroller 310 is capable of receiving internal data and/or externaldata and generating and delivering a stimulation signal to targettissues of the patient's body. For example, the controller 310 mayreceive manual instructions from an operator externally, or may performstimulation based on internal calculations and programming. Thecontroller 310 is capable of affecting substantially all functions ofthe IMD 200.

The controller 310 may comprise various components, such as a processor315, a memory 317, etc. The processor 315 may comprise one or more microcontrollers, microprocessors, etc., that are capable of performingvarious executions of software components. The memory 317 may comprisevarious memory portions where a number of types of data (e.g., internaldata, external data instructions, software codes, status data,diagnostic data, etc.) may be stored. The memory 317 may comprise randomaccess memory (RAM) dynamic random access memory (DRAM), electricallyerasable programmable read-only memory (EEPROM), flash memory, etc.

The IMD 200 may also comprise a stimulation unit 320. The stimulationunit 320 is capable of generating and delivering stimulation signals toone or more electrodes via leads. A number of leads 122, 134, 137 may becoupled to the IMD 200. Therapy may be delivered to the leads 122 by thestimulation unit 320 based upon instructions from the controller 310.The stimulation unit 320 may comprise various circuitry, such asstimulation signal generators, impedance control circuitry to controlthe impedance “seen” by the leads, and other circuitry that receivesinstructions relating to the type of stimulation to be performed. Thestimulation unit 320 is capable of delivering a controlled currentstimulation signal over the leads 122.

The IMD 200 may also comprise a power supply 330. The power supply 330may comprise a battery, voltage regulators, capacitors, etc., to providepower for the operation of the IMD 200, including delivering thestimulation signal. The power supply 330 comprises a power-sourcebattery that in some embodiments may be rechargeable. In otherembodiments, a non-rechargeable battery may be used. The power supply330 provides power for the operation of the IMD 200, includingelectronic operations and the stimulation function. The power supply 330may comprise a lithium/thionyl chloride cell or a lithium/carbonmonofluoride cell. Other battery types known in the art of implantablemedical devices may also be used.

The IMD 200 also comprises a communication unit 360 capable offacilitating communications between the IMD 200 and various devices. Inparticular, the communication unit 360 is capable of providingtransmission and reception of electronic signals to and from an externalunit 370. The external unit 370 may be a device that is capable ofprogramming various modules and stimulation parameters of the IMD 200.In one embodiment, the external unit 370 is a computer system that iscapable of executing a data-acquisition program. The external unit 370may be controlled by a healthcare provider, such as a physician, at abase station in, for example, a doctor's office. The external unit 370may be a computer, preferably a handheld computer or PDA, but mayalternatively comprise any other device that is capable of electroniccommunications and programming. The external unit 370 may downloadvarious parameters and program software into the IMD 200 for programmingthe operation of the implantable device. The external unit 370 may alsoreceive and upload various status conditions and other data from the IMD200. The communication unit 360 may be hardware, software, firmware,and/or any combination thereof. Communications between the external unit370 and the communication unit 360 may occur via a wireless or othertype of communication, illustrated generally by line 375 in FIG. 3.

The IMD 200 may also comprise a signal detection unit 340. The signaldetection unit 340 is capable of receiving the signals relating to thefirst and second electrodes and their corresponding leads. Voltagesignals and/or resultant artifact signals from the first and secondelectrodes 220, 230 (FIG. 2) may be received by the signal detectionunit 340. The signal detection unit 340 is capable of performing variousfiltering and/or amplification to condition the detected signals. Thesignal detection unit 340 detects various characteristics that may leadto the measurement of an impedance. A more detailed illustration of thesignal detection unit is provided in FIG. 4 and accompanying descriptionbelow.

The IMD 200 may also comprise an impedance unit 350. The impedance unit350 is capable of performing various comparisons of the voltages and/orother signals relating to the leads as well as to the first and secondelectrodes 220, 230. The impedance unit 350 is capable of calculatingthe impedance based upon various comparisons to determine the impedanceof a lead/electrode. A more detailed illustration and description of theimpedance unit 350 is provided in FIG. 5 and accompanying descriptionbelow. Additionally or alternatively, information necessary to performthese calculations may be provided via communication unit 360 toexternal unit 370 for computation.

Turning now to FIG. 4, a block diagram depiction of the signal detectionunit 340 in accordance with an illustrative embodiment of the presentinvention is provided. The signal detection unit 340 may comprise afilter unit 410 and an amplifier unit 420. The filter unit 410 maycomprise various signal filters, such as band-pass filters, high-passfilters, low-pass filters, etc. These filters may filter voltagesignals, current signals, etc., received by the IMD 200. The filteredsignals may be amplified and/or buffered by the amplifier unit 420. Oneor more amplifiers in the amplifier unit 420 may amplify the signalsreceived from the first electrode 220 and/or the second electrode 230.

Turning now to FIG. 5, a block diagram depiction of the impedance unit250 in accordance with an illustrative embodiment of the presentinvention is provided. The impedance unit 350 may comprise a comparatorunit 530 and an impedance calculation unit 540. The comparator unit 530may comprise various comparators that are capable of comparing numerouselectrical indications of the signal present on the first electrodeand/or the second electrode 220-230. For example, indications of thefirst impedance, such as the voltage relative to the first electrode toa reference electrode, may be compared to the voltage relative to thesecond electrode 230 to a reference electrode. Based upon thesecomparisons, an impedance comparison may be based using Ohms Law. In analternative embodiment, the comparator unit 530 may compare impedancecalculations, where the first impedance 240 may be compared to thesecond impedance 260 and/or to the third impedance 250. Alternatively,the comparator unit 530 may compare resultant artifact signals from thefirst and second electrodes 220, 230. Alternatively, measurements ofthese electrical indications may be recorded for subsequentcomputational comparison by processor 315 or external unit 370.

The impedance calculation unit 540 may comprise various circuitry tocalculate the first impedance 240, the second impedance 260 and/or thethird impedance 250. The impedance may be calculated using varioustechniques (e.g., the known current level of the test pulse delivered toan electrode being divided by the resultant voltage) to calculate theimpedance. The impedance calculation unit 540 is capable of calculatingvarious impedances of leads/electrodes associated with the IMD 200. TheIMD 200 may then use the various impedance calculations to makeassessments relating to the lead conditions and/or lead orientation.Alternatively, measurements of voltages and/or currents may be recordedfor subsequent computational determination and/or comparison ofimpedance by processor 315 or external unit 370.

Turning now to FIG. 6, a flowchart depiction of one embodiment ofperforming a lead condition assessment, in accordance with an embodimentof the present invention, is illustrated. The IMD 200 may receive acommand to perform a lead condition assessment (block 610). The commandto perform the lead condition assessment may come from an externalsource via the communication unit 360. In an alternative embodiment, theIMD 200 may be programmed to generate a signal to initiate the leadcondition assessment. In yet another alternative embodiment, apredetermined condition may trigger the initiation of the lead conditionassessment process. Upon initiation of the lead condition assessmenttest, the IMD 200 may measure the first impedance 240, which may bebetween the first electrode 220 and a reference electrode that in oneembodiment comprises the case 121 of the IMD 200 (block 620). Themeasurement of the first impedance 240 may entail delivering a signal,such as a test pulse signal, to the first electrode. The IMD 200measures the resultant voltage in response to the current test signaldelivered to the electrode 220.

Upon detection of the voltage resulting from the delivered test current,the impedance may be calculated. The IMD 200 may also measure the secondimpedance 260, which may be the impedance between the second electrode230 relative to a reference electrode (block 630). The second impedance260 may be measured by providing a test current signal to the secondelectrode 230 and measuring the resultant voltage. The impedance of thesecond electrode 230 relative to the reference electrode may then becalculated. Upon measurements of the first and the second impedances,the IMD 200 may compare the first and second impedances (block 640).This comparison may be made to determine whether the differences betweenthe first and second impedances are above a predetermined threshold(block 640). Alternatively or additionally, the comparison may be anevaluation of the ratio of impedances or the comparison of eachindividual impedance to preset limits.

Pre-determined and/or previously calculated indications of the expectedvalues of the first and second impedances may be stored in the IMD 200.The fact that the second electrode may be substantially distal to theproximally positioned first electrode 220 may result in expected and/orpre-determined impedance differences between the first and the secondimpedances 240, 260. Taking into account the expected impedancedifferential, the comparison of the actual first and second measuredimpedances may indicate an impedance difference that is above or belowan expected threshold or tolerance range. A determination is madewhether the comparison of the first and second impedances is indeedbeyond a threshold/tolerance range (block 650). Upon a determinationthat the difference between the first and second impedances are notsubstantially above or below the predetermined threshold or tolerancerange, a report of no significant lead condition problems may beprovided by the IMD 200 (block 660). This indication may be stored inthe IMD 200 and/or communicated to an external entity, such as aphysician, via the communication unit 360. Upon a determination that thecomparison of the first and the second impedances are either below orabove the predetermined range of tolerance or have excessive differencesor ratios, an indication may be provided that the electrodes and/or theleads conditions are suspect (block 670).

The indication that the lead condition is suspect may be communicated tothe external unit 370 via the communication unit 360. The substantialdifference above or below the predetermined threshold range may beindicative of the fact that the electrode path relating to the firstand/or the second electrodes 220-230 may contain a problem. A warningsignal may be provided by the IMD 200 to warn a physician that furtheranalysis and trouble-shooting may be required.

FIG. 6 also illustrates an alternative embodiment that may be employedby the IMD 200 in conjunction with the steps described above. Thesteps/paths associated with the alternative embodiment are denoted bydotted lines. Upon measuring the first and second impedances 240, 260,the IMD 200 may measure a third impedance 250, which may provide anindication of an impedance between the first and the second electrode(block 675). Upon measurements of the first, second, and thirdimpedances 240-260, the IMD 200 may perform the comparison between thefirst and the second impedances to determine if the differences aboveare below a pre-determined threshold.

Upon an indication that the difference between the first and the secondimpedances are outside the acceptable threshold/tolerance range (block650), the IMD 200 may compare the first, second and third impedances(block 685). For example, the difference between the first and thesecond impedances may be compared to the third impedance 250 in order todetermine a likely source of any significant impedance differences. Forexample, the assessment of comparison for differences between the firstand the second impedances to the third impedance 250 may provide anindication whether the source of the impedance difference is the firstelectrode 220, or the second electrode 230, and/or another factor (e.g.,a third factor). The third factor may include a physiologic change inthe impedance. For example, scar tissue or other changes in the humanbody surrounding the first and the second electrodes may cause a changein the physiologic impedance, thereby hampering the delivery ofstimulation. Therefore, based upon the comparisons of the variousimpedances, the IMD 200 may identify the likely source of thesignificant impedance differences (block 690). Upon identifying thesource of the significant impedance differences, the IMD 200 may provideinformation relating to the significant source of impedance, along withthe indication that the impedance differences between the first and thesecond impedances is above or below a predetermined threshold, asindicated by blocks 695 and 670. Therefore, the IMD 200 is capable ofproviding information relating to lead conditions and possibleindications of the cause of the impedance changes and/or changes in thelead/electrode conditions. A physician may then take corrective actionto modify the operation of the IMD 200.

Turning now to FIG. 7, a flowchart depiction of an alternativeillustrative embodiment of performing the lead condition assessment isprovided. The IMD 200 may receive a command to perform a lead conditionassessment (block 710). The command to perform the lead conditionassessment may come from an external source or from an internal source(e.g., the controller 310). Alternatively, the IMD 200 may be programmedto generate a signal to initiate the lead condition assessment process,e.g., at a regular interval such as a day, a week, or a month. Uponinitiation of the lead condition assessment test, the IMD 200 maymeasure the first impedance 240, which may be between the firstelectrode 220 and a reference electrode (block 720). The measurement ofthe first impedance 240 may include delivering a signal, such as a testpulse signal, to the first electrode. The IMD 200 measures the resultantvoltage in response to the current test signal delivered to theelectrode 220.

Upon receiving the command to perform the lead condition assessment, theIMD 200 may also measure the signal artifact on the second electrode(i.e., a first signal artifact) resulting from the test signal sent tothe first electrode (block 730). The signal artifact data may then bestored for further analysis. The IMD 200 may also perform a measurementof the second impedance (block 740). While delivering a test signal toperform the measurement of the second impedance, the resultant signalartifact on the first electrode (i.e., a second signal artifact) is alsomeasured (block 750). The IMD 200 may then perform an analysis of thedifferences in the signal artifact on the first electrode and the secondelectrode (block 760). Various characteristics, such as the amplitude,the pulse width, etc., relating to the first and second signal artifactsmay be analyzed.

Upon analysis of the signal artifacts, the IMD 200 may make adetermination whether the difference between the first and the secondsignal artifacts (i.e., an artifact differential) is above or below apredetermined threshold/tolerance range (block 770). Upon adetermination that the difference between the first and the secondsignal artifacts is within the tolerance range, a report that nosignificant condition errors are detected may be provided by the IMD 200(block 780). Upon a detection that the differences between the first andthe second signal artifacts are outside the tolerance range, the IMD 200may provide an indication that the electrodes and/or the correspondinglead conditions are suspect (block 790). Therefore, the impedancemeasurements and/or the signal artifacts resulting from energizing theelectrodes may be used to determine and assess the possible leadconditions. In an alternative embodiment, the impedance measurements ofFIG. 6, as well as the signal artifact measurements of FIG. 7, may becombined to provide further indications of possible lead conditionerrors and their possible causes.

Turning now to FIG. 8, a flowchart depiction of performing the leadorientation detection, in accordance with an illustrated embodiment ofthe present invention is illustrated. The IMD 200 may receive a commandto perform a lead orientation check or assessment (block 810). Thiscommand may be received from an external source via the communicationunit 360. Alternatively, the command to perform the lead orientationcheck may be an automated self-check that may be performed in anautomated pre-determined fashion and/or may be resultant of apre-determined triggering of a condition. Upon receiving the command toperform the lead orientation check, the IMD 200 may provide a signal,such as a current pulse signal, to the first electrode (block 820). Thepulse signal may be similar to the test signal described above. Uponsending the pulse signal to the first electrode, a resultant signalartifact on the second electrode may be measured (block 830). The signalartifact may then be recorded for further analysis (block 830).

Additionally, the IMD 200 may send a test signal to the second electrode230 (block 840). Based upon the signal sent to the second electrode 230,the IMD 200 may measure a resultant artifact on the first electrode(block 850). This data may also then be stored for further analysis.Based upon the artifact signals detected on the first and the secondelectrodes, the IMD 200 may identify the distal electrode as well as theproximal electrode by (block 860). For example, the distal electrode mayprovide a larger signal artifact on the proximal electrode as comparedto the artifact signal produced on the distal electrode as a resultantof the signal energized on the proximal electrode. The first electrode220, which is a proximal electrode being energized by a test signal, mayprovoke a smaller artifact signal on the second/distal electrode 230.Conversely, the distal electrode (the second electrode 230) beingenergized by a test signal may produce a larger artifact signal on theproximal first electrode 220. This may be true since the current flowduring each test signal pulse may be directed towards the IMD 200.Therefore, an electrode that is between the distal electrode and the IMD200, i.e., the proximal electrode (first electrode 220) may experience alarger artifact due to the current flow flowing through it.

Based on the similar reasoning, the distal electrode, i.e., the secondelectrode 230 may produce a smaller artifact due to the fact that muchof the current resultant from the energizing of the proximal electrode(the first electrode 220) flows towards the IMD 200, away from thedistal electrode 230. Therefore, the artifact level on the secondelectrode 230 (i.e., the distal electrode) may be smaller. Therefore,the signal artifact level comparisons may be used to identify theproximal electrode and the distal electrode. These findings may be thencompared with the expected indication of which electrode is the distalelectrode and which electrode is the proximal electrode (block 870).

A determination is made whether the orientation that is predetermined isequivalent to the measured distal and proximal positions of theelectrodes (block 880). Upon a determination that the lead orientationis different from the pre-determined lead orientation, the IMD 200 mayreport that the lead/electrode orientation may have been reversed (block890). Upon a determination that the measured lead orientation is thesame as the expected lead orientation, a report by the IMD 200 may beprovided indicating that the lead orientation is correct (block 895). Inthis manner, the IMD 200 is capable of automatically detecting andreporting whether the lead orientation has been inadvertently orintentionally reversed. This information may then be sent to an externalentity, such as a physician, via the communication unit 360. Thephysician may then take corrective action. Alternatively, the physicianmay not react based on various reasons, such as the fact that theresulting benefits from treatments made to date may have beensatisfactory. Accordingly, an automated check may be performed to warn aphysician that lead orientation, as originally intended, may have beenreversed.

Turning now to FIG. 9, an alternative embodiment of performing the leadorientation detection in accordance with an alternative embodiment ofthe present invention is illustrated. The IMD 200 may receive a commandto perform a lead orientation check (block 910). Upon initiation of thelead orientation check, the IMD 200 may measure the first impedance 240,which may be between the first electrode 220 and a reference electrode(block 920). The measurement of the first impedance 240 may entaildelivering a signal, such as a test pulse signal, to the firstelectrode. The IMD 200 measures the resultant voltage in response to thecurrent test signal delivered to the electrode 220. The IMD 200 may alsoperform a measurement of the second impedance (block 930). The IMD 200may then perform a comparison between the first and the secondimpedances (block 940). Based on the impedances comparison the IMD 200may identify the distal/proximal electrode in light of the expectedimpedance (block 950). This may be based upon a reasoning that thedistal electrode impedance may be higher than the impedance relating tothe proximal electrode impedance due to various reasons, such as leadlength due to additional distance of the distal electrode, intermediatetissue impedance, etc. Therefore, utilizing impedance measurements andcomparisons, an indication relating to the identification and theposition of the distal/proximal electrodes/leads may be performed (block960).

A determination is made whether the orientation that is predetermined isequivalent to the measured distal and proximal positions of theelectrodes (block 970). Upon a determination that the lead orientationis different from the pre-determined lead orientation, the IMD 200 mayreport that the lead/electrode orientation may have been reversed (block980). Upon a determination that the measured lead orientation is thesame as the expected lead orientation, a report by the IMD 200 may beprovided indicating that the lead orientation is correct (block 990). Inthis manner, the IMD 200 is capable of automatically detecting andreporting whether the lead orientation has been inadvertently orintentionally reversed. This information may then be sent to an externalentity, such as a physician, via the communication unit 360. Thephysician may then make corrective actions. Alternatively, the physicianmay not react based on various reasons, such as the fact that theresulting benefits from treatments made to date may have beensatisfactory. Using this technique, an automated check may be performedto warn a physician that lead orientation, as originally intended, mayhave been reversed.

Utilizing embodiments of the present invention, automated leadorientation detection and/or lead assessments may be performed. The IMD200 may perform these checks based on external signals, internalpre-determined timing signals, triggering of various events, etc. Basedupon the lead assessments and/or the lead orientation, correctiveactions may be performed and/or adjustments to the operation of the IMD200 may result.

The particular embodiments disclosed above are illustrative only as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown other than as describedin the claims below. It is, therefore, evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

1-21. (canceled)
 22. A method comprising: determining, at an impedanceunit of an implantable medical device (IMD) or an external unitconfigured to communicate with the IMD, a first impedance relative to afirst electrode and a reference electrode; determining, at the IMD orthe external unit, a second impedance relative to a second electrode andthe reference electrode; comparing, at the IMD or the external unit, thefirst impedance to the second impedance to determine an impedancedifference; determining, at the IMD or the external unit, a position ofthe first electrode relative to the second electrode based on at leastthe impedance difference; and providing an indication of a change inimpedance in response to determining that the impedance difference isoutside a predetermined tolerance range.
 23. The method of claim 22,further comprising: determining a third impedance, the third impedancecomprising the impedance relative to the first electrode and the secondelectrode; comparing the impedance difference to the third impedance;and identifying a source of the change in impedance based upon thecomparing of the impedance difference to the third impedance, the sourcecomprising at least one of the first electrode, the second electrode,and a physiological impedance.
 24. The method of claim 22, whereindetermining the first impedance comprises: providing a test currentsignal from the IMD to the first electrode; measuring a voltage signalacross the first electrode and the reference electrode, the voltagesignal resulting from the test current signal; and determining the firstimpedance based upon the test current signal and the voltage signal. 25.The method of claim 22, wherein determining the second impedancecomprises: providing a test current signal from the IMD to the secondelectrode; measuring a voltage signal across the second electrode andthe reference electrode, the voltage signal resulting from the testcurrent signal; and determining the second impedance based upon the testcurrent signal and the voltage signal.
 26. The method of claim 22,further comprising: measuring a first signal artifact relating to thesecond electrode, the first signal artifact resulting from a testcurrent signal being applied to the first electrode; measuring a secondsignal artifact relating to the first electrode, the second signalartifact resulting from a test current signal being applied to thesecond electrode; and comparing the first signal artifact to the secondsignal artifact to determine a signal artifact differential; whereindetermining the position of the first electrode relative to the secondelectrode is further based on the signal artifact differential.
 27. Themethod of claim 26, further comprising determining which of the firstelectrode and the second electrodes is positioned distal to thereference electrode based upon the signal artifact differential.
 28. Themethod of claim 22, further comprising determining which of the firstelectrode and the second electrode is positioned distal to the referenceelectrode based upon the comparing of the first impedance to the secondimpedance.
 29. The method of claim 28, wherein determining which of thefirst electrode and the second electrode is positioned distal to thereference electrode comprises determining whether the first impedance isgreater than the second impedance.
 30. The method of claim 28, whereindetermining which of the first electrode and the second electrode ispositioned distal to the reference electrode comprises determiningwhether the positioning of the first and second electrodes is thereverse of an expected positioning.
 31. An implantable medical device(IMD) comprising: a stimulation unit configured to deliver an electricalsignal to a patient, wherein the stimulation unit is configured to becoupled to a first electrode, wherein the stimulation unit is configuredto be coupled to a second electrode; an impedance unit configured todetermine a first impedance between the first electrode and a referenceelectrode, wherein the impedance unit is configured to determine asecond impedance between the second electrode and the referenceelectrode, wherein the impedance unit is configured to compare the firstimpedance to the second impedance to determine an impedance difference;a processor coupled to the impedance unit and configured to determine aposition of the first electrode relative to the second electrode basedon at least the impedance difference; and the impedance unit is furtherconfigured to determine whether the impedance difference is outside apredetermined tolerance range, wherein the impedance unit is furtherconfigured to provide an indication of a change in impedance in responseto determining that the impedance difference is outside thepredetermined tolerance range.
 32. The IMD of claim 31, wherein theimpedance unit is further configured to determine a third impedancebetween the first electrode and the second electrode and to compare theimpedance difference to the third impedance, wherein the processor isfurther configured to identify a source of lead condition error based onthe comparison of the impedance difference to the third impedance, thesource comprising at least one of the first electrode, the secondelectrode, and a physiological impedance.
 33. The IMD of claim 31,wherein the processor is further configured to determine which of thefirst electrode and the second electrode is positioned distal to thereference electrode based upon the comparison of the first impedance tothe second impedance.
 34. The IMD of claim 33, wherein the processor isfurther configured to determine whether the first impedance is greaterthan the second impedance.
 35. The IMD of claim 33, wherein theprocessor is further configured to determine whether the positioning ofthe first and second electrodes is the reverse of an expectedpositioning.
 36. A method comprising: determining, at an impedance unitof an implantable medical device (IMD) or an external unit configured tocommunicate with the IMD, a first impedance relative to a firstelectrode and a reference electrode; determining, at the IMD or theexternal unit, a second impedance relative to a second electrode and thereference electrode; comparing, at the IMD or the external unit, thefirst impedance to the second impedance to determine an impedancedifference; determining, at the IMD or the external unit, a position ofthe first electrode relative to the second electrode based on at leastthe impedance difference; and providing an indication of a leadcondition error in response to determining that the impedance differenceis outside a predetermined tolerance range.
 37. The method of claim 36,further comprising: determining a third impedance, the third impedancecomprising the impedance relative to the first electrode and the secondelectrode; comparing the impedance difference to the third impedance;and identifying a source of the change in impedance based upon thecomparing of the impedance difference to the third impedance, the sourcecomprising at least one of the first electrode, the second electrode,and a physiological impedance.
 38. The method of claim 36, furthercomprising determining which of the first electrode and the secondelectrode is positioned distal to the reference electrode based upon thecomparing of the first impedance to the second impedance.
 39. The methodof claim 38, wherein determining which of the first electrode and thesecond electrode is positioned distal to the reference electrodecomprises determining whether the first impedance is greater than thesecond impedance.
 40. The method of claim 38, wherein determining whichof the first electrode and the second electrode is positioned distal tothe reference electrode comprises determining whether the positioning ofthe first and second electrodes is the reverse of an expectedpositioning.
 41. The method of claim 36, further comprising: measuring afirst signal artifact relating to the second electrode, the first signalartifact resulting from a test current signal being applied to the firstelectrode; measuring a second signal artifact relating to the firstelectrode, the second signal artifact resulting from a test currentsignal being applied to the second electrode; and comparing the firstsignal artifact to the second signal artifact to determine a signalartifact differential; wherein determining the position of the firstelectrode relative to the second electrode is further based on thesignal artifact differential.